Human beings typically see using stereoscopic vision. The left and right eyes of an observer each perceives slightly different views of a scene, and the brain of the observer fuses the slightly different views into a single image that provides depth information. The depth information allows the observer to perceive the relative distance to various objects in the scene. Movies filmed with a single camera may not provide such depth information to the viewer and thus tend to look flat.
Early efforts in 3-D movie technology used anaglyphs, in which two images of the same scene, with a relative offset between them, are superimposed on a single piece of movie film, with the images being subject to complimentary color filters (e.g., red and green). Viewers donned special glasses so that one image would be seen only by the left eye while the other would be seen only by the right eye. When the brains of the viewers fused the two images, the result was the illusion of depth. In the 1950s, “dual-strip” projection techniques were widely used to show 3-D movies. Using dual-strip projection techniques, two films were projected side-by-side in synchronism, with the light from each projector being oppositely polarized. Viewers wore polarizing glasses, and each eye would see only one of the two images. More recently, active polarization has been used to distinguish left-eye and right-eye images. Left-eye and right-eye images are projected sequentially using an active direction-flipping circular polarizer that applies opposite circular polarization to the left-eye and right-eye frames. The viewer dons glasses with opposite fixed circular polarizers for each eye, so that each eye sees only the intended frames. Various other systems for projecting 3-D movies have also been used over the years.
3-D movies have been played in theatres and in home entertainment systems. The 3-D movies may be produced using stereoscopic techniques. Stereoscopic techniques create an illusion of depth from a pair of 2-D images, each of which is presented to a separate eye of a viewer. The pair of 2-D images may represent two slightly different perspectives of a scene. The slightly different perspectives may resemble the natural, binocular vision of the eyes of the viewer. By presenting 2-D images of slightly different perspectives to the right eye and to the left eye of the viewer, respectively, the viewer may perceive a three dimensional composite of the 2-D images, in which certain objects of the scene appear nearer to the viewer than other objects of the scene. That is, the brain of the viewer may merge or fuse the left and right eye images to create a perception of depth.
The degree of offset of objects in the image pair determines the depth at which the objects are perceived by the viewer. An object may appear to protrude toward the viewer and away from the neutral plane or screen when the position or coordinates of the left eye image are crossed with the position or coordinates of the right eye image (e.g., negative parallax). In contrast, an object may appear to recede or be behind the screen when the position or coordinates of the left eye image and of the right image are not crossed (e.g., positive parallax).
Some movies are filmed (in the case of live action movies) or imaged (in the case of rendered animations) in stereo for 3-D viewing. Image frames used to produce stereoscopic video (or stereo video) may be referred to as stereoscopic images. An image frame (or simply, frame) refers to an image at a specific point in time. An illusion of motion may be achieved by presenting multiple frames per second (fps) to the viewer, such as twenty-four to thirty fps. A frame may include content from a live action movie filmed with two or more cameras. A frame may also include content from a rendered animation that is imaged using two camera locations. In stereo video, stereoscopic perception results from presenting a left eye image stream and a right eye image stream to the viewer.